Method and apparatus for a control circuit responsive to an impedance coupled to a control circuit terminal

ABSTRACT

A controller includes a current sense circuit and a voltage regulation circuit. The current sense circuit generates a first signal that indicates whether a current through a sense terminal exceeds a first threshold current level, which indicates a fault condition of a power converter. The voltage regulation circuit regulates the sense terminal to a first voltage level when the current through the terminal is less than the first threshold current level and regulates the sense terminal to a second voltage level when the current exceeds the first threshold current level. The current sense circuit generates a second signal that indicates whether the current through the sense terminal exceeds a second threshold current level while the sense terminal is regulated to the second voltage level. The response circuit generates an output signal that determines a response of the controller to the fault condition based on the second signal.

REFERENCE TO PRIOR APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/553,730, filed Jul. 19, 2012, now pending, which is a continuation ofU.S. application Ser. No. 13/213,898, filed Aug. 19, 2011, now U.S. Pat.No. 8,243,480, which is a continuation of U.S. application Ser. No.12/497,366, filed Jul. 2, 2009, now U.S. Pat. No. 8,004,864, which is acontinuation of U.S. application Ser. No. 11/543,506, filed Oct. 4,2006, now U.S. Pat. No. 7,576,528. U.S. application Ser. No. 13/553,730and U.S. Pat. Nos. 8,243,480, 8,004,864 and 7,576,528 are herebyincorporated by reference.

BACKGROUND INFORMATION

1. Field of the Disclosure

The present invention relates generally to control circuits, and morespecifically, the present invention relates to control circuits that areresponsive to an impedance at a control circuit terminal.

2. Background

Integrated circuits may be used for a multitude of purposes andapplications. Many applications have cost goals that limit thefunctionality of the integrated circuit in order to meet these goals.The package in which the integrated circuit is housed can significantlycontribute to its cost. The number of pins or terminals that it uses inturn influences the cost of the integrated circuit package. The numberof pins that can be used to meet cost goals therefore often limits thenumber of features or options that can be provided to customers using anintegrated circuit.

An example of this can be appreciated with respect to an over-voltageprotection feature commonly provided by control circuits used in powerconversion applications. Depending on the customer, the desired responseto an over-voltage fault condition may be for the power converter tostop operating and require the power converter to be reset by, forexample, removing and reapplying the input voltage before the powerconverter starts to operate again. In other cases a customer may wishthe response to an over-voltage condition to be an automatic restartafter a shutdown period, an operation often referred to as auto-restart.

In order to provide customers with these different responses to the sameoperating condition, it is often necessary to manufacture two versionsof the same integrated circuit with the response to an over-voltagecondition as the only difference. This introduces additionalmanufacturing costs and overhead associated with holding inventory oftwo integrated circuit types with a single distinguishing feature.Alternatively the same integrated circuit could have multiple separateterminals to accommodate the various responses to an operatingcondition, which increases the cost of the package used to house theintegrated circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the present invention aredescribed with reference to the following figures, wherein likereference numerals refer to like parts throughout the various viewsunless otherwise specified.

FIG. 1 is a block diagram illustrating generally an example of a portionof a controller coupled to receive a current flowing through a senseterminal of the controller with an example of a current sense circuitsensing a magnitude of the current in accordance with the teachings ofthe present invention.

FIG. 2 shows generally examples of waveforms of current and voltage at asense terminal in accordance with the teachings of the presentinvention.

FIG. 3 shows generally an example of a flowchart for a controllerresponsive to an impedance coupled to a controller circuit terminal inaccordance with the teachings of the present invention.

FIG. 4 is a block diagram illustrating generally an example of a portionof a controller coupled to receive a voltage at a sense terminal of thecontroller with an example voltage sense circuit sensing the value of avoltage between the sense terminal and a reference potential inaccordance with the teachings of the present invention.

FIG. 5 shows generally waveforms of voltage and current at a senseterminal in accordance with the teachings of the present invention.

FIG. 6 shows generally an example flowchart for a controller responsiveto an impedance coupled to a controller circuit terminal in accordancewith the teachings of the present invention.

FIG. 7 shows generally an example schematic of a circuit coupled toreceive a current flowing through a sense terminal with an examplecurrent sense circuit sensing a magnitude of the current flowing throughthe sense terminal in accordance with the teachings of the presentinvention.

FIG. 8 shows generally an example schematic of a circuit coupled toreceive a current flowing through a sense terminal with a current sensecircuit sensing a magnitude of the current flowing through the senseterminal in accordance with the teachings of the present invention.

FIG. 9 shows generally an example schematic of a power converteremploying a controller comprising a circuit coupled to receive a currentflowing through a sense terminal with a current sense circuit sensing amagnitude of the current flowing through the sense terminal inaccordance with the teachings of the present invention.

FIG. 10 shows generally an example block diagram of a portion of acontroller coupled to receive a current flowing through a sense terminalof the controller with an example current sense circuit sensing amagnitude of the current flowing through the sense terminal inaccordance with the teachings of the present invention.

FIG. 11 shows generally waveforms of current and voltage at a senseterminal in accordance with the teachings of the present invention.

FIG. 12 shows generally an example flowchart for an example controllerresponsive to an impedance coupled to a controller circuit terminal inaccordance with the teachings of the present invention.

FIG. 13 shows generally an example block diagram of a portion of acontroller coupled to receive a current flowing through a sense terminalof the controller with a current sense circuit sensing a magnitude ofthe current flowing through the sense terminal in accordance with theteachings of the present invention.

FIG. 14 shows generally waveforms of current and voltage at a senseterminal in accordance with the teachings of the present invention.

FIG. 15 shows generally an example flowchart for an example controllerresponsive to an impedance coupled to a controller circuit terminal inaccordance with the teachings of the present invention.

DETAILED DESCRIPTION

Examples of apparatuses and methods for implementing a control circuitresponsive to an impedance at a control circuit terminal are disclosed.In the following description, numerous specific details are set forth inorder to provide a thorough understanding of the present invention. Itwill be apparent, however, to one having ordinary skill in the art thatthe specific detail need not be employed to practice the presentinvention. Well-known methods related to the implementation have notbeen described in detail in order to avoid obscuring the presentinvention.

Reference throughout this specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with the embodiment is included in at least oneembodiment of the present invention. Thus, the appearances of thephrases “in one embodiment” or “in an embodiment” in various placesthroughout this specification are not necessarily all referring to thesame embodiment. Furthermore, the particular features, structures orcharacteristics may be combined for example into any suitablecombinations and/or sub-combinations in one or more embodiments.

A control circuit responsive to an impedance at a control circuitterminal in accordance with the teachings of the present invention willnow be described. Embodiments of the present invention involve methodsand apparatuses to generate control circuits responsive to impedances atcontrol circuit terminals.

FIG. 1 shows generally an example block diagram of a control circuitbeing a portion of a controller in accordance with the teachings of thepresent invention. Current sense circuit 110 senses the magnitude of acurrent 122 flowing in a sense terminal 104. In another example, thedirection or polarity of current 122 through terminal 104 could bereversed in accordance with the teachings of the present invention. Asshown in the example, current 122 can be sensed at either side, 107 or114, of voltage regulation circuit 118. Depending on which side 107 or114 that current 122 is sensed, a current sense signal 108 or 113 isprovided to current sense circuit 110.

In the example, voltage regulation circuit 118 regulates the voltageV_(V) 103 between the sense terminal 104 and reference potential 102,which in this example is coupled to the controller 106 ground potentialterminal 105. In the example of FIG. 1 voltage regulation circuit 118 isa series regulator circuit. In another example, a shunt regulatorcircuit configuration could be used in accordance with the teachings ofthe present invention. In the example, the sense terminal voltage 103 isregulated to a first voltage level when the magnitude of the current 122flowing through the terminal 104 is below a first threshold value.

As shown, coupled between sense terminal 104 and an external biasvoltage V_(BIAS) 101, is an impedance block 181. In various examples,impedance block 181 could include a resistor 120, Zener diode 119,capacitor 182 or some combination thereof to make up an impedancecoupled between sense pin 104 and the source of external bias voltage101 in accordance with the teachings of the present invention. Impedanceblock 181 could also include an inductor, though as this is thought tobe less likely for practical reasons associated with the low frequencyimpedance of low cost inductors, this is not considered further. Thechoice of external impedance will be discussed in more detail withreference to FIG. 2.

In an example where impedance block 181 comprises a resistor 120, ifV_(BIAS) 101 voltage increases, the current 122 flowing in resistor 120also increases. If the magnitude of the current flowing through terminal104 reaches a first threshold current level, the value of which isdetermined by the design of current sense circuit 110, a signal 109 isprovided to the voltage regulation circuit 118, which sets a secondvoltage regulation level. This second voltage regulation level can behigher or lower than the first regulation voltage level. When thevoltage V_(V) 103 has settled at the second voltage regulation level,the current 122 is again sensed by current sense circuit 110. Themagnitude of the current 122 flowing through terminal 104 at the secondvoltage regulation level determines the output signal 112 of currentsense circuit 110 and therefore the output of the response circuit 117.

In one example, if the second voltage regulation level is lower than thefirst voltage regulation level, and if the magnitude of current 122 atthe second voltage regulation level is greater than a second thresholdcurrent level, this could indicate that a Zener diode 119 is coupled tothe sense terminal 104 instead of the resistor 120, since the slope ordynamic impedance of Zener diodes is very low once the rated Zenervoltage has been reached. If however, at the second voltage regulationlevel, the magnitude of current 122 does not exceed a second currentthreshold level, this could indicate that a resistor 120 is used insteadof the Zener diode 119. The above description assumes that the resistor120 impedance is much higher than the Zener diode 119 slope impedance.

The response of the response circuit 117 can be for example to cause thecontroller 106 to shutdown indefinitely, or latch off, if the current122 at the second voltage regulation level is greater than the secondcurrent threshold level for at least a measurement delay period. If themagnitude of current 122 does not exceed the second current thresholdlevel when voltage V_(V) 103 is regulated at the second voltageregulation level, then the response of the response circuit 117 can befor example to cause the controller 106 to shutdown only for a shortperiod and then restart automatically.

In the case of a latch off condition, in one example, controller 106 isshutdown indefinitely until a power supply voltage at a Vcc terminal 180providing power to the controller 106 is allowed to fall below a resetthreshold level in order to reset the controller 106 and allow a restartwhen the power supply voltage is reintroduced. In one example, wherecontroller 106 is used in an AC/DC power converter circuit, the powersupply voltage to the controller 106 could be allowed to go below thethreshold to reset the controller 106 by removing the AC input voltageto the power converter for a period of time in accordance with theteachings of the present invention. In one example, reset of thecontroller 106 could be achieved without needing to allow the voltage atthe Vcc terminal 180 to fall below a reset threshold level and insteadanother terminal of the controller 106 could be used to reset thecontroller 106 operation.

In one example, the source of V_(BIAS) 101 voltage could be atransformer bias winding in a power converter circuit. A rise in a biaswinding voltage could indicate a fault condition in the power converteroperation. It is therefore of great benefit to the user of a controller106 to program the response of the controller circuit to this type offault condition. In other examples the operating condition to which aresponse is generated need not be a fault condition but could be anyother operating condition where a response needs to be generated. In oneexample the operating condition could be an external shutdown signalapplied to a controller, where for example the response could be toshutdown until the controller is reset or automatically restart thecontroller after a fixed shutdown period. For the purposes of thefollowing description, an example of a fault condition is used.

FIG. 2 illustrates generally example waveforms in support of thedescription above. The waveforms of plot 200 show the variation of I_(V)201 with time t 213. The waveform of plot 290 shows the variation ofV_(V) 204 with time t 213. As shown, for a first period 225, I_(V) 207is below a first threshold level I_(TH1) 203. For period 225, V_(V) 204is regulated at first voltage level V_(V1) 205.

At time 226, I_(V) reaches the first threshold level I_(TH1) 203 andV_(V) 204 is then regulated at a second voltage level V_(V2) 206. In theexample shown, V_(V) 204 is regulated at the second voltage levelsubstantially immediately I_(V) reaches the first threshold levelI_(TH1) 203. In another example, the sense terminal could be regulatedto the second voltage level a delay period after the current flowingthrough the sense terminal reaches the first threshold current levelI_(TH1) 203. In one example I_(V) 201 rises to a new level 211 that isbelow a second threshold level I_(TH2) 202 indicating that a resistiveimpedance 120 in FIG. 1 is included in impedance block 181 and coupledto sense terminal 104. In another example I_(V) 201 rises to a highernew level 210 that is above a second threshold level I_(TH2) 202, whichindicates that a much lower impedance, such as for example Zener 119, isincluded in impedance block 181 and coupled to sense terminal 104 inFIG. 1. In the example, voltage levels V_(V1) 205 and V_(V2) 206 aresubstantially constant. In another example, voltage levels V_(V1) 205and V_(V2) 206 will vary slightly according the value of the magnitudeof current I_(V) 122 flowing through sense terminal 104. Signal 212 inFIG. 2 shows one possible practical characteristic of I_(V) 201 overtime when the second voltage regulation level V_(V2) 206 is set. In oneexample the reason for this type of characteristic could be thatcontroller 106 is a power converter controller where response circuit117 in FIG. 1 has generated an output signal 115 in response to aninitial response signal 111, as soon as I_(TH1) 203 is exceeded, thatwill cease the operation of the controller 106, which in turn ceasesoperation of the power converter in which it is used. Initial responsesignal 111, if used, is therefore applied regardless of the impedance ofthe impedance block 181 coupled to sense terminal 104 and is thereforenot dependent on the impedance coupled to sense terminal 104. Ifcontroller 106 ceases operation, in a power converter circuit, capacitor121 in FIG. 1 will start to discharge. The value of I_(V) will thereforebegin to fall as illustrated by curve 212 in FIG. 2. In a practicalimplementation, a delay period 209 may be included to ensure immunity tonoise before the response circuit 117 commands an indefinite shutdown,or latching off, of controller 106. It is therefore important to ensurethat capacitor 121 is large enough to maintain a value of I_(V) 201greater than second threshold I_(TH2) 202, to allow response circuit 117to provide the correct output signal 115 at the end of measurement delayperiod 209 in accordance with the teachings of the present invention.

The operation described above allows the controller 106 to sense ormeasure an impedance coupled to the sense terminal 104 when a magnitudeof the current flowing through terminal 104 exceeds a threshold value.The response generated by the control circuit is therefore dependent onthe value of the impedance coupled to the sense terminal in accordancewith the teachings of the present invention.

In the example of FIG. 1 a single component, either 120 or 119, isincluded in impedance block 181 and coupled to sense terminal 104.However in other examples, the impedance coupled to the sense terminal104 could be made up of more than one component. In that case, the powersupply controller response would be responsive to or dependent on theimpedance of the complete circuit of impedance block 181 coupled to thesense terminal 104 in accordance with the teachings of the presentinvention.

As described above, in one example, current sense circuit 110 couples asecond signal 111 to response circuit 117 as an indication for examplethat the first current threshold I_(TH1) has been exceeded and that asecond phase of detecting an impedance coupled to the sense terminal 104is starting. In one example this second signal 111 could generate aninitial response from circuit 117, which is independent of the impedanceof the circuit coupled to terminal 104. In one example where controller106 is a power converter controller, the initial response could be tocease energy transfer to an output of the power converter to ensure thepower converter is protected immediately when a fault condition isindicated by the fact that the first current threshold I_(TH1) has beenexceeded. In one example, this initial response signal 111 would then befollowed by signal 112 coupling to response circuit 117 to determine thefinal response to the fault condition in accordance with the teachingsof the present invention.

The examples described above have been limited to a single secondcurrent threshold level I_(TH2). However, in one example one or moreadditional current sense levels could be sensed by current sense circuit110 to generate a plurality of response circuit outputs as illustratedwith the plurality of response outputs 116 in FIG. 1.

The examples described above have been limited to a single secondvoltage regulation voltage level V_(V2) 206. However, in one example,when the current 122 flowing through the sense terminal 104 exceeds afirst threshold, a plurality of voltage levels can be implemented whichin one example can be alternating voltage levels to provide analternating voltage level over time at sense terminal 104. The presenceof an alternating voltage level at sense terminal 104 provides thecapability to also detect for example a capacitive external impedance182 coupled to sense terminal 104. In general, a capacitive impedancesensing scheme of this type would be more complex to implement and inthe alternative embodiments discussed below, sensing of capacitiveimpedances is therefore not discussed. It is however understood that thegeneral principle can be applied to any of the embodiments discussedbelow. It is therefore understood that in one example, sensing theimpedance coupled to the sense terminal comprises detecting currentflowing through the sense terminal at a plurality of voltage levels onthe sense terminal.

FIG. 3 shows generally an example flowchart of the operation of anexample controller in accordance with the teachings of the presentinvention. In block 301, V_(V) is regulated to the first regulationvoltage level V_(V1). In block 302 the current flowing through the senseterminal, I_(V), is monitored to establish whether it has reached afirst threshold value, I_(TH1). If the current flowing through the senseterminal, I_(V), reaches I_(TH1), in block 303 an initial response isimplemented if required by the application of the controller. In block304, V_(V) is regulated to a second voltage level V_(V2). In block 305,I_(V) is compared to a second current threshold value I_(TH2). If I_(V)reaches I_(TH2), block 306 generates a response.

In the example flowchart shown in FIG. 3, a plurality of sense terminalthreshold current levels are used to compare to the current flowing inthe sense terminal as illustrated in blocks 307 and 309. Where, in block309, I_(V) is compared to an nth threshold current value I_(THn)generating one of response (n−1) or response n in blocks 310 and 311respectively. In the flowchart of FIG. 3, the plurality of senseterminal threshold levels are compared to the current flowing in thesense terminal sequentially. It is understood that in a circuitimplementation, the comparisons could be made simultaneously.

Although not shown so as not to obscure the teachings of the presentinvention, it is also possible in an example to use a plurality ofvoltage regulation thresholds to monitor the change in current flowingin the sense terminal with each. In this way, the impedance of thecircuit coupled to the sense terminal could be characterized over anumber of different voltage regulation thresholds in accordance with theteachings of the present invention.

FIG. 4 shows generally a block diagram of an example control circuitbeing a portion of a controller in accordance with the teachings of thepresent invention. In the example, voltage sense circuit 410 senses themagnitude of a voltage, V_(V) 403, between sense terminal 404 andreference potential terminal 405. Current 422 is regulated to a firstcurrent value determined by variable current source 418, when voltageV_(V) 403 is below a threshold value and a second value determined byvariable current source 418, when voltage V_(V) 403 reaches thethreshold value. Coupled between sense terminal 404 and an external biasvoltage V_(BIAS) 401, is an impedance block 481. In various examples,impedance block 481 could include for example a resistor 420, a Zenerdiode 419 or some combination thereof to make up an impedance coupledbetween sense pin 404 and the source of external bias voltage 401.

In one example, variable current source 418 conducts a first value ofsubstantially zero current when voltage V_(V) is below the firstthreshold voltage value such that I_(V) 422 is also substantially equalto zero. Under these conditions, the voltage V_(V) is substantiallyequal to V_(BIAS) 401. In one example variable current source 418conducts a finite second value of current when voltage V_(V) reaches thefirst threshold voltage value. Under these conditions, the voltage V_(V)is reduced since a voltage drop is generated across impedance 481. Thechange in voltage V_(V) is dependent on the value of the impedances 481.In one example, if a low resistance values is used for resistor 420, thechange in voltage when the sense terminal current 422 is regulated tothe second current value is less than an example where a high resistancevalue is used for resistive element 420.

In the example shown in FIG. 4, if resistive element 420 is replacedwith Zener diode 419, the characteristics of the circuit change in thatthe Zener diode is substantially an open circuit when the voltage acrossZener diode 419 is below its rated threshold voltage. When the voltageacross Zener diode 419 reaches its rated threshold voltage it presents avery low impedance for any further increase in the voltage across it. Assuch, in FIG. 4 when a Zener diode 419 is used in place of resistor 420,the voltage V_(V) 403 shows very little change when variable currentsource 418 regulates I_(V) 422 to the second current level in accordancewith the teachings of the present invention.

In a practical circuit implementation, variable current source 418 couldactually include two current sources that are switched in and out ofcircuit depending on the value of the voltage V_(V) according to thedescription above. The output of voltage sense circuit 410 and responsecircuit 417 share many aspects with the operation of circuit 100 in FIG.1 in accordance with the teachings of the present invention.

FIG. 5 illustrates generally example waveforms in support of thedescription above of circuit 400. The waveforms of plot 500 show thevariation of V_(V) 501 with time t 513. The waveform of plot 590 showsthe variation of I_(V) 504 with time t 513. For a first period 525,V_(V) 507 is below a first threshold level V_(TH1) 503. For period 525,I_(V) 504 is regulated at first current value I_(V1) 506, which in oneexample could be substantially zero. At time 526, V_(V) reaches thefirst threshold level V_(TH1) 503 and I_(V) 504 is regulated at secondcurrent value I_(V2) 505. In the example shown, I_(V) 504 is regulatedat the second current value substantially immediately V_(V) reaches thefirst threshold level V_(TH1) 503. In another example the sense terminalcould be regulated to the second current value a delay period afterV_(V) reaches the first threshold voltage level V_(TH1) 503. In oneexample V_(V) 501 is reduced to a new level 511 that is above a secondthreshold level V_(TH2) 502. In another example V_(V) 501 is reduced toa lower new level 510 that is below the second threshold level V_(TH2)502 indicating that a higher impedance such as resistor 420 is coupledto sense terminal 404 in FIG. 4. Range arrow 514 indicates the range ofdifferent voltages V_(V) 501 that could result when I_(V) 505 isregulated to the second value I_(V2), depending on the impedance of thecircuit coupled to the sense terminal in accordance with the teachingsof the present invention.

In one example, controller 406 in FIG. 4 is a power convertercontroller. In one example an initial response signal 411 is coupled toresponse circuit 417 to generate an initial response, which for examplecould be to cease the transfer of energy to an output of the powerconverter in order to protect the power converter immediately from afault condition is indicated by the voltage V_(V) 403 reaching the firstthreshold level. The initial response signal is therefore independent ofthe impedance coupled to terminal 404. Plots 515 and 516 in FIG. 5illustrate example characteristics for voltage V_(V) 501 over time afteran initial response. It is important therefore that a final responsegenerated in response to signal 412 in FIG. 4 is provided before voltageV_(V) 501 decays too far. In the example of plot 515 in FIG. 5, overtime the voltage V_(V) 501 will decay below V_(TH2) 502 and thereforelead to an incorrect response if sensed after time 527.

FIG. 6 shows an example flowchart of the operation of an examplecontroller in accordance with the teachings of the present invention. Inblock 601, I_(V) is regulated to the first regulated current valueI_(V1). In block 602, the voltage between the sense terminal and areference potential, V_(V), is monitored to establish whether it hasreached a first threshold value, V_(TH1). When V_(V) reaches V_(TH1),block 603 implements an initial response if required by the applicationof the controller. In block 604, I_(V) is regulated to a second currentvalue I_(V2). In block 605, V_(V) is compared to a second voltagethreshold value V_(TH2). If V_(V) is less than V_(TH2), block 606generates a first response output.

In the example flowchart of FIG. 6 a plurality of sense terminalthreshold voltage levels are used to compare to the voltage V_(V)between the sense terminal and reference potential terminal asillustrated in blocks 607 and 609. Where, in block 609, V_(V) iscompared to an nth threshold voltage level V_(THn) generating one ofresponse (n−1) or response n in blocks 610 and 611 respectively. In theflowchart of FIG. 6, the plurality of sense terminal voltage levels arecompared to the voltage V_(V) between the sense terminal and referencepotential terminal sequentially. It is understood that in a circuitimplementation, the comparisons could be made simultaneously.

Although not shown so as not to obscure the teachings of the presentinvention, it is also possible to use a plurality of current regulationvalues to monitor the change in voltage V_(V) at the sense terminal witheach. In this way, the impedance of the circuit coupled to the senseterminal could be characterized over a number of different currentregulation thresholds. It is therefore understood that in one examplesensing the impedance coupled to the sense terminal comprises detectingthe voltage level at the sense terminal at a plurality of currentsflowing through the sense terminal.

FIG. 7 is a schematic showing generally a portion of an examplecontroller 738 in accordance with the teachings of the presentinvention. As shown, the example schematic of FIG. 7 shares many aspectsof its operation with the block diagram example shown of FIG. 1. Avoltage regulation circuit 753 is coupled to sense terminal 704, whichis coupled to receive a current I_(V) 722. Current sense circuit 754 iscoupled to sense the magnitude of the current flowing through senseterminal 704 similar to current sense element 114 in FIG. 1. In oneexample, the current sense element, which is shown as a separate item114 in FIG. 1, is included as part of current sense circuit 754 in FIG.7. Current sense circuit 754 couples signals 745 and 746 to responsecircuit 717, which in turn couples one or more response signals 715 to apart of the controller 738 not shown so as not to obscure the teachingsof the present invention. In one example, signal 745 is an initialresponse signal that may generate an initial response from responsecircuit 717 independent of the impedance of circuitry coupled to senseterminal 704.

In the following description, all example voltages are expressedrelative to reference potential 703 unless otherwise stated. Undernormal operating conditions, switch 732 is closed and voltage source 733is therefore coupled to apply 2V to the gate 790 of P channel MOSFET791. In operation, the source 792 of MOSFET 791 is regulated to thevalue of the voltage at gate 790 plus the threshold voltage of theMOSFET, which is typically in the order of 1 volt for an integratedMOSFET. Since source 792 is coupled to sense terminal 704, the voltageat the sense terminal 704 is therefore regulated as a function of thevoltage applied to the gate 790 of MOSFET 791.

As shown in the example, the current flowing through the sense terminal704 is mirrored from transistor 734 through transistors 735 and 739. Inone example, the current mirror including transistors 734, 735 and 739is a 1 to 1 to 1 current mirror as indicated by the ratios expressed inlabel 752. In other examples different ratios could be used to step downthe sense terminal current to lower values for example to reduce theinternal consumption of the controller 738.

In the example, the reflected sense terminal current 722 flowing intransistor 735 is compared to a first threshold current level I_(TH1)737, supplied from internal supply rail 740, using inverter gate 793.Whenever the sense terminal current 722 is less than I_(TH1) 737, thevoltage at node 749 is high. The signal 750 from node 749 is applied toswitch 732 to keep it on as described above. If, however, the senseterminal current 722 exceeds I_(TH1) 737, the voltage at node 749 goeslow. Switch 732 is turned off and the output signal 751 of inverter gate793 goes high. The output signal 751 is applied to switch 730, whichcouples voltage source 731 to gate 790 of MOSFET 791.

In one example, voltage source 731 has a value of 1.5V. Compared tovoltage source 733, this results in the voltage at the sense terminal704 dropping by approximately 0.5 volts. This corresponds to V_(V2) 206in the example shown in FIG. 2. When output signal 751 of inverter gate793 goes high, switch 750 is also switched on. The current flowing intransistor 739 is then compared to a second threshold current levelI_(TH2) 741 using inverter gate 742. If the current flowing in senseterminal 704 is greater than second threshold current level I_(TH2) 741,then output of inverter gate 742 goes high. The output of logic gate 744also goes high and signal 746 is applied to response circuit 717 asdescribed with reference to the description of the example circuit andplots of FIG. 1 and FIG. 2.

FIG. 8 shows generally a detailed schematic of a portion of an examplecontroller 800 in accordance with the teachings of the presentinvention. The example circuit shares many aspects of the operation withthe example schematic of FIG. 7 described above. In the descriptionbelow, all example voltages are expressed with reference to referencepotential 802 unless otherwise stated.

As shown, response circuit 817 is coupled to sense terminal 804 throughthe operation of current sense circuit 810 and voltage regulationcircuit 818. In the example, the response circuit 817 coupled to beresponsive to the impedance of an external circuit coupled to senseterminal 804 when the current 822 flowing through sense terminal 804exceeds a threshold value. When the current flowing through senseterminal 804 is below the threshold value, switch 856 is closed. Thevoltage at sense terminal 804 is then regulated to a value substantiallyequal to the voltage of voltage source 858.

In the example, the circuitry shown in FIG. 8 coupling switch 856 tosense terminal 804 is more complex than the example circuit couplingswitch 732 to sense terminal 704 in FIG. 7 in order to remove theinfluence of switch threshold voltages. However, the operation of thiscircuitry is not necessary for a controller to benefit from theteachings of the present invention and is therefore not described hereso as not to obscure the teachings of the present invention.

Continuing with the example shown in FIG. 8, signal 813 is coupledbetween voltage regulation circuit 818 and current sense circuit 810 inthe way signal 113 in FIG. 1 couples to the current sense circuit 110.The current flowing through sense terminal 804 is mirrored to transistor862. In one example, the ratio of this current mirror steps the senseterminal current down by a factor of 6 as indicated by label 865, tolimit the internal current consumption of controller 800. Current source864 sets a maximum current level that can flow in transistor 861.

The current flowing in transistor 862 is mirrored through the currentmirror made up with transistors 854 and 852. In one example, thiscurrent mirror also includes a resistor 850 and capacitor 851 coupled tofilter the current flowing in transistor 852 so as to improve noiseimmunity of the circuit. Current source 855 has a similar function tocurrent source 737 in FIG. 7 and sets the level of a first thresholdcurrent.

In operation, if the current flowing through the sense terminal 804leads to a current flow in transistor 852 that exceeds the currentflowing in current source 855 then the output of inverter gate 886changes from high to low. The signal 809 is coupled to switches 856 and857 to regulate the voltage level on the sense terminal 804 to besubstantially equal to voltage source 859 when the output of invertergate 886 goes from high to low. In one example, voltage source 859 has avalue of 2.5V. In one example, the signal 809 coupled to switches 856and 857 is the same as signal 812, which couples current sense circuit810 to response circuit 817 as an indication that the current flowingthrough sense terminal 804 has exceed a first current threshold level.Signal 812 provides information to response circuit 817 to enable aninitial response to the fact that the current flowing through senseterminal 804 has reached the first current threshold level. This signal812 is therefore applied regardless of the impedance of an externalcircuit coupled to sense terminal 804.

In one example signal 809 is applied to the input of inverter gate 867,which turns on switch K2 866. In addition, in one example, signal 809 isalso applied to delay circuit 853, which couples an output signal 871 toswitch K1 868 to an on state when signal 809 goes from high to low for adelay period determined by the output of inverter gate 872 as will bedescribed below. As shown, the circuitry between delay circuit 853 andswitch 868 includes a latch, which includes cross-coupled NAND gatescoupled to a NOR gate. In one example therefore, switch K1 868 isswitched on when the current flowing through sense terminal 804 reachesa first current threshold value determined by the value of currentsource 855 as described above.

In the example, the current flowing in switch K1 868 is set at the valueof current source 870, which in one example is 250 μA. The reason thatswitch 868 is included in one example is related to the nature of theexternal circuitry that may be coupled to sense terminal 804. In oneexample where a Zener diode similar to 119 in FIG. 1 is coupled betweenthe sense terminal 804 and an external bias voltage such as 101 in FIG.1, the increase in current flowing through sense terminal 804 couldincrease very significantly when the regulation voltage of senseterminal 804 is changed when switch 856 turns off and switch 857 turnson as described above. Under these conditions, in one example,additional current source 870 is required to ensure transistor 860 involtage regulation circuit 818 conducts less current to avoid thevoltage on sense terminal 804 rising significantly, which would corruptthe impedance measurement that will be performed when the voltageregulation level on sense terminal 804 is set by voltage source 859 inaccordance with the teachings of the present invention.

In the example, when switch 866 is on, transistor 863 is directlycoupled to current source 869. Current source 869 and current source 870therefore set the threshold of a second current level, which if exceededwill change the polarity of the output of inverter gate 872 from low tohigh. As noted by label 873, a high level or ‘1’ output from invertergate 872 will set the delay period of delay circuit 853 to infinity,which in one example will result in switch 868 being on indefinitelysince this condition indicates that the external circuitry coupled tosense terminal 804 has low impedance. If, however, the current flowingin sense terminal 804 is below the second threshold current, then delaycircuit 853 turns off switch 868 after a delay period which in oneexample is 500 nsecs.

In one example, the signal 811, which is used to control switch 868, isalso coupled to response circuit 817. In one example signal 811determines the response of controller 800 dependent on the impedance ofthe external circuit coupled to sense terminal 804. In one example, ifsignal 811 remains high for a period longer than the delay period set bydelay circuit 888, the controller 800 is latched into an off state,requiring a cycling of power to the controller, which in one example isprovided at a Vcc terminal 880, to restart operation. In one example, ifsignal 811 is low after a delay period set by delay circuit 853, thecontroller 800 is turned off for a first period of time and isautomatically restarted after the first period and turned on for atleast a second period of time.

FIG. 9 shows generally an example schematic 900 of an AC to DC powerconverter circuit employing a controller 906 in accordance with theteachings of the present invention. As shown, the power converter iscoupled to receive an AC input voltage 993 and output a DC voltage 992.The example schematic 900 shows a flyback power converter configuration.Bias voltage V_(BIAS) 901 is applied across capacitor 921. An optionaloutput over-voltage protection (OVP) circuit 991 is coupled betweencapacitor 921 and sense terminal 904 of controller 906.

In the example, a detection circuit 991 is included, which uses a Zenerdiode 919 similar to Zener diode 119 in FIG. 1. However, in thispractical implementation, a resistor 940 is added. In the example, Zenerdiode 919 is used to isolate the sense terminal 904 from the voltageacross capacitor 921 under normal operating conditions. This isnecessary since resistor 941 is also coupled to the sense terminal 904and provides information to the controller 906 regarding the inputvoltage 993 which would be corrupted by current flowing through OVPcircuit 991. The Zener diode 919 only conducts when a fault conditionoccurs that allows the voltage across capacitor 921 to increase to alevel that the Zener threshold voltage of Zener diode 919 is reached. Inthe example therefore, Zener diode 919 is used regardless of therequired response and the resistor 901 value is chosen to determine thetype of response required of controller 906. When sense terminal 904current I_(V) 922 is below a first threshold current value, the senseterminal 904 is regulated to a first voltage level relative to referencepotential terminal 905. When sense terminal current I_(V) 922 reachesthe first threshold value, the sense terminal 904 is regulated to asecond voltage level relative to reference potential terminal 905.

In the example, the value of the sense terminal current I_(V) 922 isdetected when the sense terminal 904 is regulated to the second voltagelevel relative to reference potential terminal 905. The controller 906is coupled to respond depending on the value of the sense terminalcurrent I_(V) 922 when the sense terminal 904 is regulated to the secondvoltage level relative to reference potential terminal 905.

In common with the previous example circuits described above therefore,controller 906 measures an impedance of a circuit coupled to the senseterminal 904 when a magnitude of a current flowing through the senseterminal 904 reaches a threshold value in accordance with the teachingsof the present invention. The controller 906 response is then dependenton the measured impedance of the circuit coupled to the sense terminal904 in accordance with the teachings of the present invention.

In one example, one response could be to shutdown the controller 906operation such that energy is no longer delivered to power converteroutput 992 until the AC input voltage 993 is removed allowing controller906 to reset and restart operation when the AC input voltage 993 isagain introduced. In one example, another response could be to shutdownthe controller 906 operation such that energy is no longer delivered topower converter output 992 for a period of time and then automaticallyrestart controller 906 operation without it being necessary to remove ACinput voltage 993. As the name implies, this over-voltage protection maybe used in power converter circuits to protect load circuitry that willbe coupled to DC output 992, from being damaged due to a power converterfault condition that leads to the voltage appearing at DC output 992rising above its normal regulated value.

The option of shutting down controller 906 indefinitely, or a latchingshutdown, until AC input voltage 993 is removed and reintroduced orautomatically restarting after a shutdown period, normally requireseither two separate controller terminals or separate controller designsthat must be chosen by the customer, both of which add cost to themanufacture of the controller and power converter.

It will be noted that in the practical implementation of the exampleshown in FIG. 9, an additional resistor 941 is coupled to the senseterminal 904. In the example, resistor 941 is used to sense an inputvoltage to the power converter and allow sense terminal 904 to alsoprovide a protection feature called input or line over-voltage shutdown.A single sense terminal 904 can therefore be used to sense over-voltagefault conditions in the AC input voltage 993 as well as sensing outputover-voltage fault conditions of output voltage 992 in accordance withthe teachings of the present invention. Although the controller 906therefore also effectively measures the impedance coupled to the senseterminal 904 including this additional resistor 941, the value of theresistor 941 impedance is generally very high compared to that of outputOVP circuit 991 and therefore has very little influence on the operationof the controller 906 in accordance with the teachings of the presentinvention.

FIG. 10 shows generally a block diagram of an example control circuitbeing a portion of a controller in accordance with the teachings of thepresent invention. The example circuit of FIG. 10 shares many aspects ofits operation with the example block diagram of FIG. 1. However, thevalue of the sense terminal voltage V_(V) 1003 is not regulated to asecond voltage level when the current flowing through terminal 1004exceeds a first threshold value. Instead current sense circuit 1010includes a timer that times a measurement delay period from the timewhen the current flowing through terminal 1004 reaches a first thresholdvalue. The current flowing through the sense terminal 1004 is thensensed once the measurement delay is completed. In the example, thesignal 1012 to response circuit 1017 is only applied once themeasurement delay period is complete. In one example therefore, theinitial response signal 111 in FIG. 1 is no longer required in FIG. 10.

In one example where controller 1006 is a power converter controlleremployed in a power converter circuit, the power supply controller wouldcontinue to operate when the current I_(V) 1022 flowing through senseterminal 1004 reaches the first current threshold level. In an examplepower converter circuit of the type shown in FIG. 9 for example, theoperation described above would lead to the V_(BIAS) 901 voltagecontinuing to rise when the current I_(V) flowing in terminal 904exceeds a first threshold level because the power converter does notimplement an initial response and would continue to operate until themeasurement delay period is complete.

FIG. 11 illustrates generally example waveforms in support of thedescription above of the block diagram in FIG. 10. The waveforms of plot1100 show the variation of I_(V) 1101 with time 1113. The waveform ofplot 1190 shows the variation of V_(V) 1104 with time 1113. As shown,for a first period 1125, I_(V) 1101 is below a first threshold levelI_(TH1) 1103. At time 1126, I_(V) reaches the first threshold levelI_(TH1) 1103. In one example, no change is made to V_(V) 1104, whichremains regulated at V_(V1) and therefore V_(V1) and V_(V2) aresubstantially equal. In one example I_(V) 1101 continues to rise at arate dependent on the impedance of the external circuitry coupled tosense terminal 1004 in FIG. 10. After measurement delay period 1109, thecurrent I_(V) is sensed at time 1128.

In one example shown by plot 1107, I_(V) at time 1128 is below a secondthreshold level I_(TH2) 1102, which indicates that impedance block 1081includes a resistive impedance 1020 in FIG. 10 coupled to sense terminal1004. In another example shown by plot 1127, I_(V) 1101 rises to ahigher new level that is above a second threshold level I_(TH2) 1102,which indicates that a much lower impedance, such as Zener 1019, iscoupled to sense terminal 1004 in FIG. 10. In this way, the impedance ofthe external circuitry coupled to sense terminal 1004 in FIG. 10 issensed in accordance with the teachings of the present invention. In oneexample a capacitor 1150 may be coupled between sense terminal 1004 andreference potential terminal 1005. In one example, capacitor 1150 isused to set up a time constant to influence the characteristic of senseterminal current 1022 over time. The response of controller 1006 isdependent on the value of the measured impedance of the externalcircuitry coupled to sense terminal 1004 as described with reference tothe previously described examples.

FIG. 12 shows generally a flowchart of the operation of an examplecontroller benefiting from the teachings of the present invention asdescribed with reference to FIG. 10 and FIG. 11 above. In block 1201,V_(V) is regulated to the first regulation voltage level V_(V1). Inblock 1202 the current flowing in the sense terminal, I_(V), ismonitored to establish whether it is above a first threshold value,I_(TH1). If the current flowing in the sense terminal, I_(V), reachesI_(TH1), in block 1203 a measurement delay is implemented. In block1204, I_(V) is compared to a second current threshold value I_(TH2) oncemeasurement delay period is complete. If I_(V) is greater than I_(TH2),block 1205 generates a first response.

In the flowchart of FIG. 12 a plurality of sense terminal thresholdcurrent levels are used to compare to the current flowing in the senseterminal as illustrated in blocks 1206 and 1208. Where, in block 1208,I_(V) is compared to an nth threshold current value I_(THn) generatingone of response (n−1) or response n in blocks 1209 and 1210respectively.

FIG. 13 shows generally a block diagram of an example control circuitbeing a portion of a controller in accordance with the teachings of thepresent invention. The example circuit shares many aspects of itsoperation with the example block diagram of FIG. 10. However, currentsense circuit 1310 includes a timer that times a delay period, dt, fromthe time when the current flowing through terminal 1304 reaches a firstthreshold value to the time the current flowing through terminal 1304reaches a second threshold value I_(TH2). The delay period is thencompared to one or more threshold values to determine the response ofresponse circuit 1317. In the example, the signal 1312 to responsecircuit 1317 is only applied once the current I_(V) 1322 flowing throughsense terminal 1304 has reached the second threshold value I_(TH2).

In one example where controller 1306 is a power converter controlleremployed in a power converter circuit, the power supply controller wouldcontinue to operate when the current I_(V) 1322 flowing through senseterminal 1304 exceeds the first current threshold level. In a powerconverter circuit of the type shown in FIG. 9 for example, the operationdescribed above would lead to the V_(BIAS) 901 voltage continuing torise when the current I_(V) flowing in terminal 904 exceeds a firstthreshold level because the power converter does not implement aninitial response and would continue to operate until the current flowingin terminal 904 is greater than a second threshold current valueI_(TH2).

FIG. 14 illustrates generally example waveforms in support of thedescription above of the block diagram in FIG. 13. The waveforms of plot1400 show the variation of I_(V) 1401 with time 1413. The waveform ofplot 1490 shows the variation of V_(V) 1404 with time 1413. For a firstperiod 1425, I_(V) 1401 is below a first threshold level I_(TH1) 1403.At time 1426, I_(V) reaches the first threshold level I_(TH1) 1403. Inone example, no change is made to V_(V) 1404, which remains regulated atV_(V1) 1405 and therefore V_(V1) and V_(V2) are substantially equal.

In one example I_(V) 1401 continues to rise at a rate dependent on theimpedance of the external circuitry coupled to sense terminal 1304 inFIG. 13. In one example shown by plot 1407, I_(V) takes a timedt_(HIGHIMPEDANCE) 1429 to reach a second threshold level I_(TH2) 1402indicating that a high impedance, for example resistive impedance 1320in FIG. 13 is coupled to sense terminal 1304. In another example shownby plot 1427, I_(V) takes a shorter time dt_(LOWIMPEDANCE) 1430 to reacha second threshold level I_(TH2) 1402 indicating that a low impedance,such as for example Zener impedance 1319 in FIG. 13 is coupled to senseterminal 1304.

Therefore, the impedance of the external circuitry coupled to senseterminal 1304 in FIG. 13 is sensed in accordance with the teachings ofthe present invention. In one example capacitor 1350 is used to set up atime constant to influence the characteristic of sense terminal current1322 over time. The response of controller 1306 is dependent on thevalue of the measured impedance of the external circuitry coupled tosense terminal 1304 as described with reference to the examplesdescribed above.

FIG. 15 shows generally a flowchart of the operation of a controller inaccordance with the teachings of the present invention as described withreference to FIG. 13 and FIG. 14 above. In block 1501, V_(V) isregulated to the first regulation voltage level V_(V1). In block 1502,the current flowing through the sense terminal, I_(V), is monitored toestablish whether it has reached a first threshold value, I_(TH1). Ifthe current flowing through the sense terminal, I_(V), reaches I_(TH1),in block 1503 a time measurement is started. In block 1504, I_(V) iscompared to a second current threshold value I_(TH2). If I_(V) reachesI_(TH2), block 1505 measures the time elapsed, dt, between the currentI_(V) reaching the first threshold current I_(TH1) and reaching thesecond threshold current level I_(TH2). In block 1506, elapsed time dtis compared to a first time elapse threshold. If elapsed time dt isgreater than a first elapsed time threshold dt_(TH1) to generate a firstresponse.

In the flowchart of FIG. 15 a plurality of elapsed time thresholds areused to compare to the measured elapsed time dt, as illustrated inblocks 1508 and 1510. Where, in block 1510, dt is compared to an nthelapsed time threshold dt_(THn) generating one of response n or response(n+1) in blocks 1511 and 1512 respectively.

FIGS. 10 to 15 illustrate examples where a current flowing through asense terminal is sensed to generate a response. In this respect, theexample block diagrams of FIG. 10 and FIG. 13 are similar to the exampleblock diagram of FIG. 1. It should be noted, however, that thetechniques discussed in FIGS. 10 to 15 are equally applicable to thetechnique introduced in FIG. 4 where a voltage at a sense terminal issensed to generate a response. In this case, the first and secondcurrent thresholds of FIGS. 10 to 15 would be replaced by first andsecond voltage thresholds to determine a response to a voltage at asense terminal exceeding a first threshold voltage level in accordancewith the teachings of the present invention.

In the foregoing detailed description, the method and apparatus of thepresent invention have been described with reference to specificexemplary embodiments thereof. It will, however, be evident that variousmodifications and changes may be made thereto without departing from thebroader spirit and scope of the present invention. The presentspecification and figures are accordingly to be regarded as illustrativerather than restrictive.

What is claimed is:
 1. A controller for use in a power converter, thecontroller comprising: a sense terminal to be coupled to an impedanceblock of the power converter, wherein the impedance block is external tothe controller and is to be coupled between the sense terminal and asource of a voltage of the power converter; a current sense circuitcoupled to the sense terminal to sense a magnitude of a current flowingthrough the sense terminal and to generate a first signal that indicateswhether the magnitude of the current exceeds a first threshold currentlevel, wherein the magnitude of the current exceeding the firstthreshold current level indicates a fault condition of the powerconverter; a voltage regulation circuit coupled to the current sensecircuit and to the sense terminal, wherein the voltage regulationcircuit regulates the sense terminal to a first voltage level inresponse to the first signal indicating that the magnitude of thecurrent flowing through the sense terminal does not exceed the firstthreshold current level, and regulates the sense terminal to a secondvoltage level in response to the first signal indicating that themagnitude of the current flowing through the sense terminal exceeds thefirst threshold current level, wherein the current sense circuitgenerates a second signal indicating whether the magnitude of thecurrent exceeds a second threshold current level while the senseterminal is regulated to the second voltage level; and a responsecircuit coupled to the current sense circuit to generate an outputsignal that determines a response of the controller to the faultcondition, wherein the response of the controller is a first responsewhen the second signal indicates that the magnitude of the currentthrough the sense terminal does not exceed the second threshold currentlevel while the sense terminal is regulated to the second voltage level,and wherein the response of the controller to the fault condition is asecond response when the second signal indicates that the magnitude ofthe current exceeds the second threshold current level while the senseterminal is regulated to the second voltage level.
 2. The controller ofclaim 1, wherein the first response is one response selected from thegroup consisting of shutting down the controller until the controller isreset, and automatically restarting the controller after a shutdownperiod.
 3. The controller of claim 2, wherein the second response is theother response selected from the group consisting of shutting down thecontroller until the controller is reset, and automatically restartingthe controller after a shutdown period.
 4. The controller of claim 1,wherein the second voltage level is less than the first voltage level.5. The controller of claim 1, wherein the source of voltage of the powerconverter is a transformer bias winding of the power converter, whereina rise in voltage of the transformer bias winding indicates the faultcondition.
 6. The controller of claim 1, wherein whether the magnitudeof the current through the sense terminal exceeds the second thresholdcurrent level while the sense terminal is regulated to the secondvoltage level is responsive to whether the impedance block is aresistive impedance or a non-resistive impedance.
 7. The controller ofclaim 1, wherein whether the magnitude of the current through the senseterminal exceeds the second threshold current level while the senseterminal is regulated to the second voltage level is responsive towhether the impedance block includes a Zener diode.
 8. A controller foruse in a power converter, the controller comprising: a sense terminal tobe coupled to an impedance block of the power converter, wherein theimpedance block is external to the controller and is to be coupledbetween the sense terminal and a source of a voltage of the powerconverter; a voltage sense circuit coupled to the sense terminal tosense a magnitude of a voltage at the sense terminal and to generate afirst signal that indicates whether the magnitude of the voltage exceedsa first threshold voltage level, wherein the magnitude of the voltageexceeding the first threshold voltage level indicates a fault conditionof the power converter; a variable current source coupled to the voltagesense circuit and to the sense terminal, wherein the variable currentsource regulates a current through the sense terminal to a first currentvalue in response to the first signal indicating that the magnitude ofthe voltage at the sense terminal does not exceed the first thresholdvoltage level, and regulates the current through the sense terminal to asecond current value in response to the first signal indicating that themagnitude of the voltage at the sense terminal exceeds the firstthreshold voltage level, wherein the voltage sense circuit generates asecond signal indicating whether the magnitude of the voltage exceeds asecond threshold voltage level while the current through the senseterminal is regulated to the second current value; and a responsecircuit coupled to the voltage sense circuit to generate an outputsignal that determines a response of the controller to the faultcondition, wherein the response of the controller is a first responsewhen the second signal indicates that the magnitude of the voltage atthe sense terminal does not exceed the second threshold voltage levelwhile the current through the sense terminal is regulated to the secondcurrent value, and wherein the response of the controller to the faultcondition is a second response when the second signal indicates that themagnitude of the voltage exceeds the second threshold voltage levelwhile the current through the sense terminal is regulated to the secondcurrent value.
 9. The controller of claim 8, wherein the first responseis one response selected from the group consisting of shutting down thecontroller until the controller is reset, and automatically restartingthe controller after a shutdown period.
 10. The controller of claim 9,wherein the second response is the other response selected from thegroup consisting of shutting down the controller until the controller isreset, and automatically restarting the controller after a shutdownperiod.
 11. The controller of claim 8, wherein the second current valueis greater than the first current value.
 12. The controller of claim 8,wherein the source of voltage of the power converter is a transformerbias winding of the power converter, wherein a rise in voltage of thetransformer bias winding indicates the fault condition.
 13. Thecontroller of claim 8, wherein whether the magnitude of the voltage atthe sense terminal exceeds the second threshold voltage level while thecurrent through the sense terminal is regulated to the second currentvalue is responsive to whether the impedance block is a resistiveimpedance or a non-resistive impedance.
 14. The controller of claim 8,wherein whether the magnitude of the voltage at the sense terminalexceeds the second threshold voltage level while the current through thesense terminal is regulated to the second current value is responsive towhether the impedance block includes a Zener diode.